Cellular Aspects of Body Electrolytes and Water 21 



excretion of Na+. The mechanism is not known; presumably 

 the active transport processes are sensitive to the concen- 

 trations of Na+ and K+, or more probably to the relative 

 proportions of these ions, in the cell. 



The erythrocyte is a highly specialized cell, and it would 

 not be correct to assume that all cells of the body, or even the 

 majority, are based on a similar physiological plan so far as the 

 maintenance of salt and water content is concerned. The 

 striated muscle fibre has been studied very thoroughly, and it 

 may well be that this is far nearer to being a "typical cell", 

 so that we may now consider its main features from the pre- 

 sent point of view. The main point of difference between the 

 muscle cell and the erythrocyte lies in the low contents of Gl- 

 and HCO3-, these anions being replaced by organic anions 



Fibre Membrane Extracellular 



Fluid 



K+ A 



Na+ CI 



Fig. 4. The muscle fibre. 

 (A~^indiffusible organic anions). 



that apparently cannot diffuse across the plasma membrane; 

 schematically the situation is as in Fig. 4 where A" represents 

 these indiffusible anions. The system would be osmotically 

 stable were the membrane impermeable to Na+, i.e. the rest 

 of the ions, K+, CI", HCO3-, would distribute themselves 

 across the membrane in such a way that equal osmotic 

 activities would exist on both sides. Actually the cell mem- 

 brane is permeable to Na+, and the reason why the Na+, K+ 

 and CI- ions do not redistribute themselves is because an 

 active extrusion of Na+, as fast as it penetrates, maintains an 

 effective impermeability to Na+. There is no need to postulate 

 an active accumulation of K+ in this case since, owing to the 

 high concentration of impermeable anions in the cell, the 

 extrusion of a Na+ ion must be associated with the penetra- 

 tion of a K+ ion, in the interests of electrical neutrality. Once 

 again, then, the cell may maintain equilibrium with its 



